Natural gas contains as main components thereof lower hydrocarbons (having 1 to 5 carbon atoms in a molecule as far as this application is concerned), of which methane is a typical one. In recent years, techniques for manufacturing synthesis gas containing hydrogen and carbon monoxide as main components thereof from natural gas have been developed and improved to a great extent. Synthesis gas serves as source material for manufacturing various products by means of C1-chemistry and also as source material for manufacturing clean fuels such as methanol, synthetic gasoline and dimethyl ether (DME) and hence the reforming technology of producing synthesis gas from natural gas is an important technology that provides the basis for effective utilization of natural gas.
Known reforming techniques for producing synthesis gas from natural gas include steam reforming, auto thermal reforming (ATR) and catalytic partial oxidation (CPDX), of which steam reforming is a technique of converting natural gas into synthesis gas that contains hydrogen and carbon monoxide as main components thereof by adding steam to natural gas and causing the steam-added natural gas to pass through a reaction tube arranged in a heating furnace and filled with a reforming catalyst. However, this technique is accompanied by a problem that the above described reaction is an endothermic reaction and thermal energy needs to be massively supplied to the reaction system from outside and that a gigantic reaction apparatus is required to manufacture synthesis gas on a large scale by means of this technique. ATR, on the other hand, is a technique of burning part of the natural gas that is supplied as source gas by means of a burner with oxygen or air added thereto and causing the carbon dioxide and the moisture (steam) existing in the produced hot combustion gas to react with the unburned methane remaining in the source natural gas in a reforming catalyst layer. This technique provides an advantage that the heat generated as a result of the combustion reaction is utilized in the reforming reaction (which is an endothermic reaction) so that no thermal energy is required to be supplied from outside. However, this technique is accompanied by a problem that an upper part of the catalyst layer is exposed to hot gas and hence it can easily be degraded and additionally that the reaction system can hardly be run under economically optimal conditions because steam needs to be supplied excessively for the purpose of prolonging the service life of the burner.
Finally, CPDX is a technique obtained by replacing the combustion process of using a burner of ATR with a combustion process of using a catalyst. The practical advantage of CPDX lies in that a combustion reaction and a reforming reaction are conducted simultaneously and concurrently so that it can prevent only an upper part of the catalyst layer from becoming excessively hot unlike ATR where a combustion reaction and a reforming reaction are conducted sequentially and stepwise. However, to make this technique feasible, it is necessary to either realize “direct catalytic partial oxidation (D-CPDX)” of suspending the process of burning the lower hydrocarbons in the source natural gas at the stage where the lower hydrocarbons are oxidized to produce hydrogen and carbon monoxide so that the lower hydrocarbons may not be completely burnt out to produce carbon dioxide and moisture or develop a catalyst that can make a combustion reaction and a reforming reaction proceed simultaneously and concurrently. In the past, the inventors of the present invention succeeded in developing such a catalyst and paving a way to practical realization of CPDX (see PTL 1).